fix: [build].defines 未被传入 P1689 模块扫描，导致条件 import 与 scan_overrides 冲突 #296

复现步骤

1. 创建如下项目：

   src/main.cpp:

   #ifdef TEST_USE_MODULES
   import std;
   import lib;
   #else
   #include <print>
   #endif
   
   int main(int argc, char* argv[]) {
       std::println("Hello from hello!");
       printHello();
       return 0;
   }

   src/lib.cppm:

   #ifndef TEST_USE_MODULES
   module;
   #include <print>
   #endif
   
   export module lib;
   
   #ifdef TEST_USE_MODULES
   import std;
   #endif
   
   export void printHello() {
       std::println("Hello world in lib.cppm!");
   }

   mcpp.toml:

   [package]
   name        = "hello"
   version     = "0.1.0"
   description = "A modular C++23 package"
   license     = "Apache-2.0"
   
   [build]
   defines = ["TEST_USE_MODULES"]
   
   [scan_overrides."src/lib.cppm"]
   provides = ["lib"]
   imports = ["std"]
   
   [scan_overrides."src/main.cpp"]
   imports = ["std", "lib"]

2. 运行 mcpp build。

实际行为

构建失败，报错：

error: module-graph divergence in obj/lib.m.o:
  planned : provides [lib] imports [std]
  compiler: provides [lib] imports [<none>]
  The compiler's P1689 scan disagrees with the planner's assumption
...
error: module-graph divergence in obj/main.o:
  planned : provides [<none>] imports [lib, std]
  compiler: provides [<none>] imports [<none>]

期望行为

[build].defines 应该被脱糖为 -D<x> 并传递给 P1689 模块扫描和普通编译命令，使条件 import 被正确识别，构建成功。

环境

• mcpp 版本：2026.7.27.1
• OS：Windows（根因跨平台）
• 工具链：llvm@20.1.7